1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a reflective plate having a peak and depression structure to enhance optical efficiency of an LCD and a fabrication method thereof.
2. Description of the Related Art
In general, the cathode ray tube (CRT) has been the most popular display device but it is very inconvenient to use because of its large size and heavy weight as compared to display area.
Accordingly, a thin flat panel display was developed which can be installed and used anywhere due to its thinness despite its large display area. The thin flat panel display is being substituted for the CRT, especially, a thin film transistor liquid crystal display (TFT-LCD). The TFT-LCD has a better resolution than other display devices and a response speed as fast as a CRT in displaying a moving picture.
The operation of such a TFT-LCD will be described briefly. When a pixel is switched by a thin film transistor, the switched pixel controls the light transmittance of a lower light source. Most switching elements are amorphous silicon thin film transistors (a-Si:H TFT) whose semiconductor layer is formed of amorphous silicon. This is because that the amorphous silicon thin film can be formed at a low temperature on a large insulating substrate such as an inexpensive glass substrate.
A general LCD operates such that an image is displayed using the light generated from a light source called backlight. However, such an LCD is a very inefficient optical modulator in that only 3 to 8% of the light that is incident from the backlight is transmitted. When it is assumed that the transmittance of two polarizing plates provided on an upper plate and a lower plate of the LCD is 45%; the transmittance of two glass substrates of the upper plate and the lower plate is 94%; the aperture ratio of the TFT array and the pixel is 65%; the transmittance of a color filter is 27%, the overall light transmittance of the LCD is calculated to be about 7.4%. Similarly, because the actual light power emitted from the LCD display image is merely about 7% of the light power emitted from the backlight, the backlight power should be increased as much as needed to achieve a high luminance LCD. As a result, the backlight needs more power. Thus, when an LCD is employed in a portable apparatus such as a notebook computer or a mobile communication terminal, a battery has been used to provide sufficient backlight power.
However, the use of a battery increases the weight of the portable device, for instance, a notebook computer or a mobile communication terminal, and also has a limitation that it cannot be used for a long time. To overcome this problem, reflective LCDs have been studied in which a backlight is not employed.
The reflective LCD is structured to reflect external light by substituting opaque reflective material for the pixel electrodes formed of transparent electrodes in a conventional transmissive LCD. As a result, because the reflective LCD is operated only using natural light without any backlight, the power consumed by a backlight is saved and the reflective LCD may be used for a long time on battery power. The aperture ratio of the reflective LCD is better than the conventional LCD employing a backlight.
Meanwhile, a user may use the reflective LCD where there is not enough natural light or artificial light. In other words, the reflective LCD can be used where natural light may be used or in an office in which artificial light may be used, but it cannot be used in a dark environment in which there is no natural light.
Therefore, recently, a transflective LCD that combines the advantage of a reflective LCD using ambient light and the advantage of a transparent LCD using a backlight has been studied and researched.
Referring to FIGS. 1 and 2, the configuration and fabrication process of a conventional reflective LCD will be described.
FIG. 1 illustrates a plan view of a conventional reflective LCD schematically and FIG. 2 illustrates a cross-sectional view of the thin film transistor area of the LCD taken along the line I-I of FIG. 1 schematically.
Here, FIG. 1 shows only a lower substrate on which the thin film transistor and the pixel region are prepared. FIG. 2 also shows an upper substrate having a color filter with the lower substrate of the reflective LCD.
A plurality of data lines 156 for applying data signals to a source electrode 161 and a plurality of gate lines 155 for applying gate signals to a gate electrode 121 are arranged in a matrix configuration on a glass substrate 111 of the lower substrate of the conventional reflective LCD. Regions defined by the crossing of the data lines 156 and the gate lines 155 correspond to pixel regions for displaying images. In the reflective LCD, a pixel electrode 181 serving as a reflective plate is formed on the pixel region.
As shown in FIG. 2, a gate electrode 121 of conductive material such as metal is formed on the transparent glass substrate 111. A gate insulating layer 130 of silicon nitride (SiNx) or silicon oxide (SiOx) covers the gate electrode 121. On the gate insulating layer 130 covering the gate electrode 121, an active layer 141 of amorphous silicon is formed. On the active layer 141, an ohmic contact layer 151, 152 of impurity-doped amorphous silicon is formed. On the ohmic contact layer 151, 152, source and drain electrodes 161 and 162 of conductive material such as metal are formed. The source and drain electrodes 161 and 162 form a thin film transistor (T) together with the gate electrode 121.
On the source and drain electrodes 161 and 162, a passivation layer 170 of silicon nitride (SiNx), silicon oxide (SiOx), or organic insulator is formed. The passivation layer 170 has a contact hole 171 exposing the drain electrode 162. The passivation layer 170 may be formed by sequentially depositing a first passivation film of SiNx or the like and a second passivation film of BCB or the like.
On the pixel region of the passivation layer 170, a pixel electrode 181 of transparent conductive material may be formed. The pixel electrode 181 may be connected with the drain electrode 162 through the contact hole formed in the passivation layer 170. The pixel electrode 181 also serves as a reflective plate so as to display an image. Although not shown in the drawings, on the pixel electrode 181, an alignment layer of material such as polyimide aligned in a predetermined direction may be formed.
Here, the gate electrode 121 may be connected with the gate line 155, and the source electrode 161 may be connected with the data line 156. The gate line 155 may be arranged substantially perpendicular with the data line 156 to define the pixel region.
Over the lower substrate having the aforementioned structure, an upper substrate may be arranged with a predetermined interval. The upper substrate includes a black matrix 120 formed to correspond to the thin film transistor of the lower substrate, to prevent light from leaking in areas other than the pixel region.
Below the black matrix 120, a color filter 131 of red (R), green (G) and blue (B) is sequentially and repeatedly formed. In the color filter 131, one color corresponds to one pixel region. The color filter 131 may be formed by a dying method, a printing method, a pigment dispersion method, an electrodeposition method, or the like.
Afterwards, below the color filter 131, a common electrode 140 made of transparent conductive material may be formed. Below the common electrode 140, a second alignment layer (not shown) made of material such as polyimide having an alignment surface aligned in a predetermined direction may be formed.
In the general reflective LCD configured as described above, the light that is incident through the upper substrate passes through liquid crystal layer 190, is reflected by the reflecting surface, passes through the liquid crystal layer 190 and the upper substrate sequentially, and exits the LCD.
In the conventional reflective LCD as described above, the light that is incident into the device from an external light source situated at a predetermined direction is reflected to the side opposite to the light source so that the viewing angle becomes narrower.
FIG. 3 is a graph illustrating the variation in reflection intensity versus the reflecting angle when a reflective plate is formed as a flat mirror without a scattering layer in the conventional LCD. The maximum reflection intensity may be found at a reflection angle of 30° with respect to the light that is incident at an incident angle of −30°. The plate shows that the maximum reflection intensity appears at about 30° when viewed with reference to a reflecting surface contacting with the liquid crystal layer. This is because the light with an incident angle of 30° enters into the LCD that has a high refraction unlike air and thus the light is refracted. The light refracted in the LCD is reflected by the mirror reflecting surface and reflected and outputted at the same reflection angle as the incident angle. The reflection angle is considered to be 30° because the incident angle is considered to be 30° in the general reflective LCD. The light that is incident from the side is reflected at an output angle of 30° in the opposite direction with the incident angle with respect to a normal direction.
Accordingly, because the reflected light cannot be found at the front reflection angles (output angle of 0° to 10°) the conventional reflective LCD having a mirror reflecting surface cannot function as a display device. Accordingly, there is a need for technologies to direct the reflected light toward the front main viewing environment of the user. Peaks and depressions are formed on the reflecting surface of the conventional general reflective LCD in order to scatter the reflected light in various directions.
FIG. 4 is a schematic plan view of a reflective LCD having a reflective plate with a peak and depression structure and FIG. 5 is a sectional view taken along the line II-II of FIG. 4 and shows a thin film transistor region of the reflective LCD;
FIG. 4 shows a lower substrate on which the thin film transistor and the pixel region are prepared. FIG. 5 shows also an upper substrate having a color filter with the lower substrate of the reflective LCD.
A plurality of data lines 256 applying a data signal to a source electrode 261 and a plurality of gate lines 255 applying a gate signal to a gate electrode 221 are formed in matrix on a glass substrate 211 on the lower substrate of the conventional reflective LCD. An area made by the crossing of the data lines 256 and the gate lines 255 becomes a pixel region to display an image. In a reflective LCD, a pixel electrode 281 with a structure of peaks and depressions that serves as a reflective plate is formed on this pixel region.
As shown in FIG. 5, a gate electrode 221 made of conductive material such as metal is formed on a transparent glass substrate 211 and a gate insulating layer 230 made of silicon nitride (SiNx) film or a silicon oxide (SiOx) film covers the gate electrode 221. An active layer 241 made of amorphous silicon is formed on the gate insulating film 230 of the gate electrode 221. An ohmic contact layer 251 and 252 made of impurity-doped amorphous silicon is next formed thereon. Source and drain electrodes 261 and 262 made of conductive material such as metal are formed on the ohmic contact layer 251 and 252, and the source and drain electrodes 261 and 262 form a thin film transistor T with the gate electrode 221.
A passivation layer 270 made of a silicon nitride (SiNx) film, a silicon oxide (SiOx) film or an organic insulator is formed on the source and drain electrodes 261 and 262, and the passivation layer 270 has a contact hole 271 to expose the drain electrode 262.
A pixel electrode 281 made of transparent conductive material is formed on the pixel region of the upper portion of the passivation layer 270, and the pixel electrode 281 is connected to the drain electrode 262 through a contact hole 271. To display an image in a reflective LCD, the pixel electrode 281 serves as a reflecting surface. The gate electrode 221 is connected to a gate line 255, and the source electrode 261 is connected to a data line 256. The gate line 255 and the data line 256 are substantially parallel to each other to define pixel region.
An upper substrate is formed above the lower substrate configured as described above and spaced from the lower substrate. A black matrix 220 is formed on the upper substrate corresponding to the portion of the thin film transistor under the transparent glass substrate 210 to prevent light from leaking in an area other than pixel region.
A color filter 231 is formed under the black matrix 220 and three colors red, green and blue are formed in order on the color filter 231. One color corresponds to one pixel region.
Subsequently, a common electrode 240 made of transparent conductive material is formed on the lower portion of the color filter 231. A second alignment layer which is made of material such as polyimide and whose surface is aligned in some direction is formed on the lower portion of the common electrode 240 (not shown).
In the general reflective LCD configured as described above, the light that is incident through the upper substrate passes through the liquid crystal layer 290, is reflected by the reflecting surface, and passes the liquid crystal layer 290 and the upper substrate successively exits the LCD. On the pixel region of the reflective LCD configured as above, the reflective plate with a peak and depression structure is formed. When the reflective plate of the reflective LCD is formed with a peak and depression structure 282, the pattern of the peak and depression structure 282 may be formed in a regular configuration or in an irregular configuration in another embodiment.
When the incident light from outside the LCD is reflected and projected by the peak and depression structure 282 of the reflecting surface formed on the pixel region of the reflective LCD, the incident light can be reflected with various angles due to the peak and depression structure 282 so that light scattering is induced.
FIGS. 6 and 7 are schematic partial sectional views illustrating the light scattering effect in the reflective LCD that has a reflective plate with the peak and depression structure. If light is incident from the side of the LCD with a predetermined incident angle (generally about 30°), the incident light is refracted due to a refraction index difference between the liquid crystal layer and air while passing through the liquid crystal layer. The refracted light is reflected diffusively by the reflecting surface of the peak and depression structure, passes through the liquid crystal layer and the upper substrate successively, and then exits the LCD.
The light reflected out of the LCD is not limited to reflections to the opposite side of the LCD, but the light is reflected in various directions so that the improved reflection intensity can be obtained at the front side that is the typical user location.
Referring to FIG. 7, with a viewing angle range between 0° and 10° to the normal direction that is the typical viewing angle of the user, the light that is incident from the outside with an incident angle of 30° is diffusively reflected by the reflecting surface of the peak and depression structure. The refraction index of air is n1=1.0 and the refraction index of the LCD is n2=1.5.
FIGS. 8A and 8B are a graph illustrating reflection intensity versus the reflection angle of the reflective LCD that has a conventional mirror type reflective plate and a graph illustrating reflection intensity versus the reflection angle of the reflective LCD that has a conventional reflecting surface with a peak and depression structure, respectively.
In FIG. 8A, the reflection intensity has a maximum value at the reflection angle of about 30° with respect to light that is incident with an incident angle of 30°. This is because light with an incident angle of about 30° enters into the LCD that has a high refraction index unlike air. The light refracted in the LCD is reflected by the mirror type reflecting surface and exits from the LCD with a reflection angle that is the same as the incident angle. Accordingly, the reflected light cannot be emitted at the front reflection angles (output angle 0-10°) corresponding to the typical main user location.
FIG. 8B is a graph illustrating the reflection intensity versus the reflection angle of the reflective LCD that has a conventional reflecting surface with the peak and depression structure. Comparing this with the reflection intensity characteristic of the mirror type reflecting surface described above, the reflective LCD has some amount of reflection intensity at a front reflection angle of about 0° to about 10° corresponding to the typical user location.
Here, integration ratios of the functions in the graphs of FIGS. 8A and 8B are the same as each other. The factor to determine the characteristic of the graph is in the peak and depression structure of the reflecting surface to allow the light that is incident to be scattered in various angles by the light scattering effect.
Accordingly, for efficient light reflection of the reflecting surface, it is desirable to make the reflecting surface have the structure of peaks and depressions. An embodiment of forming the reflecting surface will be described referring to FIGS. 9 and 10.
FIGS. 9A through 9D show schematically a peak and depression formation method using a one-layer process to form a reflective plate of a reflective LCD that has a conventional reflecting surface with the peak and depression structure.
Referring to FIG. 9A, first, the substrate 300 is coated with a photosensitive resin film 310 (for example, polymer resin) to form the peak and depression structure using a process such as a spin coating, a roll coating, or the like.
As shown in FIG. 9B, a diffraction mask 320 that has a plurality of slits formed at predetermined locations is aligned over the photosensitive resin film 310, and ultraviolet radiation (denoted by arrows on the drawings) is irradiated from above.
As shown in FIG. 9C, when developing the photosensitive resin film 310 the peaks and depressions have a height difference due to the photosensitive resin film 310 being exposed at different exposure intensities due to the diffraction mask 320. This results in a plurality of peak patterns 330 on the substrate 300.
Through a curing bake process, the plurality of peak patterns 330 of the photosensitive resin film are softened. During the softening process, the thick upper portion is melted and flows down to finally form hemi-spherical peaks 340 as shown in FIG. 9D.
However, the one-layer processing method as described above has a precision problem with the exposure method and exposure mask.
According to recent research, the optimal front reflection ratio may be obtained when a ratio of height to radius of the finally formed peak is 1:10 more or less and the radius of the peak is formed at 4˜5 μm more or less.
However, in the reflective LCD of the one-layer process method, if the peak pattern of the reflecting surface is about 5 μm, because the height of the peak and depression should be controlled to be 0.5 μm, the process margin is very little.
When the peak and depression pattern is formed randomly to avoid the interference of light generated between peaks while the light is reflected in the process, the gap between the peaks is generated due to the random arrangement and the density of the peaks is lowered.
When forming the peaks of the reflecting surface as described above, the peak scatters light due to the unevenness of the hemi-spherical surface to the external incident light. However, because the portion between the peaks is flat, there still exists a problem that reflection intensity is high only in a specific direction.
To overcome the disadvantages of special exposure such as diffraction exposure, there are methods as reflecting surface formation method.
FIGS. 10A through 10E show schematically a peak and depression formation using a two-layer method to form a reflecting surface of a reflective LCD that has a conventional reflecting surface with the peak and depression structure.
Referring to FIG. 10A, first, the substrate 400 is formed with a photosensitive resin film 410 (for example, polymer resin) to form the peak and depression structure using the method such as spin coating, roll coating, or the like. The thickness of the photosensitive resin film 410 is 2 to 3 μm.
As shown in FIG. 10B, a diffraction mask 420 that has a plurality of slits formed at predetermined locations is aligned over the photosensitive resin film 410, and ultraviolet radiation (denoted by arrows on the drawings) is irradiated from above.
As shown in FIG. 10C, when developing the photosensitive resin film 410 the peaks and depressions have a height difference due to the photosensitive resin film 410 being exposed at different exposure intensities due to the diffraction mask 420. This results in a plurality of peak patterns 430 on the substrate 400.
Subsequently, through the curing bake process, the plurality of peak patterns 430 of the photosensitive resin film are softened to form curved hemi-spherical patterns 440 of peaks and depressions as shown in FIG. 10D. The photosensitive resin film peak and depression pattern 430 can be implemented to have different heights.
If photo acryl is coated on the peak and depression pattern formed by the process and hardened, as shown in FIG. 10E, the photo acryl layer 460 flows along the curve of the peaks and depressions, fills the depressions and finally decreases the height difference between the peaks and depressions so that the desired peak and depression structure is formed. However, the two-layer process is sensitive to the thermal characteristic of the second formed acryl layer.
When the peak and depression pattern is formed randomly to avoid the interference of the light generated between peaks and depressions while the light is reflected in the process, the gap between the peaks is generated due to the random arrangement and the density of the peaks is lowered. Therefore the reflection efficiency is lowered.